Whiskey barrels at the Jack Daniel's distillery

An aging barrel is a barrel used to age wine or distilled spirits such as whiskey, brandy, or rum. Specialty beers are also sometimes aged in barrels which were previously used in aging harder spirits, thus imparting characteristic and distinctive flavors to the beer; lambic beers are aged in used wine barrels.

When a wine or whiskey/whisky ages in a barrel, small amounts of oxygen are introduced as the barrel lets some air in (compare to microoxygenation where oxygen is deliberately added). Oxygen enters a barrel when water or alcohol is lost due to evaporation, a portion known as the "angels' share". In an environment with 100% relative humidity, very little water evaporates and so most of the loss is alcohol, a useful trick if one has a wine with very high proof. Most wines are topped up from other barrels to prevent significant oxidation, although others such as vin jaune are not.

Wine aged in small new oak barrels takes on some of the compounds in the barrel, such as vanillin and wood tannins. The presence of these compounds is dependent on many factors, including the place of origin, how the staves were cut and dried, and degree of "toast" applied during manufacture. After roughly three years, most of a barrel's flavor compounds have been leached out and it is well on its way to becoming "neutral."

The tastes yielded by French and American species of oak are slightly different, with French oak being subtler, while American oak gives stronger aromas [1]. To retain the desired measure of oak influence, a winery will replace a certain percentage of its barrels every year, although this can vary from 5 to 100%. Some winemakers use 200% new oak, where the wine is put into new oak barrels twice during the aging process.

Bulk wines are sometimes flavored by soaking oak chips in them instead of barrel aging.

Barrels used for aging are typically made of oak, but chestnut and redwood are not unknown.